This invention relates to amphibious hovercraft vehicles. More particularly, this invention is to a hovercraft vehicle having a waterjet/water propeller drive system capable of being raised above the water-air interface by a skirt cell system for traversing dry land, soft mud, marsh, and tidal flat areas.
Hovercrafts, also known as Air-Cushioned Vehicles (ACVs), are typically marine craft that ride on a pressurized cushion of air. The air cushion serves to separate the vehicle from the surface over which it operates allowing freer movement and greatly decreasing the hydrodynamic drag. The lower drag allows much greater speeds to be achieved at lower drive power when compared to conventional displacement hulls. Powered lift fans are needed to maintain the flow of pressurized air under the vehicle due to the flow of air escaping from the cushion, and the amount of air escaping is typically minimized by including some sort of flexible fabric system, known as a “skirt”, in order to better contain the air cushion by conforming more closely to irregular operating surface, such as waves or uneven terrain.
The separation of the hovercraft vehicle from the surface makes it potentially capable of truly amphibious operation. This means it has the ability to traverse dry land as well as open water, in addition to soft mud, marsh, or tidal flat areas impassible to any other type of vehicle. However, amphibious capability is only achievable if the means of propulsion is not restricted to only water-borne use. This means that systems incorporating water propellers and/or waterjet drive propulsion cannot be used in amphibious hovercrafts, as these will not be effective for land or very shallow water operations. Furthermore these water drive mechanisms will generally need to be located below the vehicle's normal waterline, where they would drag along the ground during land transit and impede movement and damage the mechanisms. As a result, amphibious hovercrafts typically employ some means of air propulsion, usually air propellers, or sometimes air is ducted from the lift fans and directed to achieve thrust for propulsion.
The disadvantage of the air propulsion systems normally employed with amphibious hovercrafts is that they are very inefficient and noisy. Air propellers also require a large amount of vehicle space, and because of vibration, exposure to sand and ingestion of water, maintenance issues arise.
By contrast propulsion by waterjet/propeller systems is much more efficient, quiet, and compact, and consequently, this means of propulsion is being used reliably in many commercial marine vehicle applications. Higher fuel efficiency means that a heavy-lift hovercraft can carry more payload in lieu of fuel, or can carry a payload for greater distances. Waterjet/propeller propulsion systems are more “harbor friendly” in that the vehicle can operate in restricted waterways next to piers and other crafts without blasting them with high velocity air and waterspray as well as undue noise.
However, conventional designs for hovercraft vehicles are presently incapable of effectively incorporating the advantages of both hovercraft and waterjet/propeller propulsion systems. This operational inability is due to design limitations inherent in conventional hovercraft design. Normally the gap beneath the hovercraft, also known as “cushion height”, is adjustable by varying the air flow and pressure to the cushion. Changing the cushion height is easily done by either adjusting the fan speed or by controlling the flow with adjustable vanes inserted into the air ducts feeding fan air to the cushion. If a conventional hovercraft were to include water propulsion means such as a waterjet intake located on the hull underside, the vehicle could potentially increase its cushion height to the point where no contact would be made with the land surface for overland operations. Land transit would not be impeded provided that the vehicle employed alternate means for land propulsion such as air jets or mechanical traction devices. However, to put the waterjet intake into contact with the water to allow it to draw in water would necessarily require that the hull underside be in close proximity with the water surface. Such close contact would impede the waterborne operational capability of the hovercraft in all but the calmest water conditions, as wave interaction with the hull would impede the water transit and negate the advantages of the air cushion, i.e. the cushion height would not be large enough.
Thus, in accordance with this inventive concept, a need has been recognized in the state of the art for a system for adjusting trim of a hovercraft vehicle to maintain the majority of the underside of the hovercraft vehicle's hull a suitable distance from the water surface for waterborne operations and to raise up the hovercraft where the waterjet intakes or water propellers are located for land operations by selectively inflating or deflating strategically located skirt cells to control the elevation of the waterjet intakes or water propellers.